This invention relates to a novel process for the production of compounds which may be used in the production of azetidine-2-carboxylic acid (AzeOH).
L-Azetidine-2-carboxylic acid (L-AzeOH) is known to be useful in the synthesis of inter alia high molecular weight polypeptides and in particular as an analogue of the well known amino acid proline.
This amino acid is of limited availability from natural sources and consequently the development of an efficient and economic synthetic method for its production is desirable.
The formation of racemic AzeOH derivatives by cyclisation of halobutyric acid derivatives has been known for many years.
For example, Fowden (in Biochem. J. (1956) 64, 323) employed barium hydroxide and Duplan et al (in Bull. Soc. Chem. Chim. France (1968) 4079) employed sodium hydroxide to effect the cyclisation of 4-amino-2-halobutyric acids.
Similarly, the synthesis of racemic AzeOH benzyl ester derivatives from benzyl 2,4-dibromobutyrate was first reported by Phillips and Cromwell (in J. Heterocyclic Chem. (1973) 10, 795). In this publication, it is stated that 1-benzhydryl-2-AzeOH benzyl ester may be prepared by refluxing benzyl 2,4-dibromobutyrate in the presence of benzhydrylamine and spectral grade acetonitrile for 24 hours.
More recently, European patent application EP 827 954 discloses the formation of N-(alkylbenzyl)-AzeOH esters by reaction of butyric acid ester derivatives with optically-active alkylbenzylamines. The butyric acid esters are substituted at the 2- and 4-positions with leaving groups (such as halo). Benzyl, and certain alkyl, 2,4-dichlorobutyrates, as well as benzyl, and certain alkyl, 2,4-dibromobutyrates are specifically mentioned.
None of the above-mentioned documents describe the cyclisation of alkylphenyl, or alkyl, 2-bromo-4-chlorobutyrates in the presence of a benzylamine, so forming N-benzyl AzeOH alkylphenyl, or alkyl, esters. We have found that four-membered rings comprising the azetidine-2-carboxylate moiety may be obtained surprisingly efficiently, and in a surprisingly good yield, by way of just such a process.
According to a first aspect of the invention there is provided a process for the production of an optionally substituted N-benzyl AzeOH alkylphenyl, or alkyl, ester which process comprises the reaction of an optionally substituted alkylphenyl, or an optionally substituted alkyl, 2-bromo-4-chlorobutyrate with an optionally substituted benzylamine, which process is referred to hereinafter as xe2x80x9cthe process of the inventionxe2x80x9d.
There is further provided a process for the preparation of a compound of formula I; 
wherein
R1 represents optionally substituted lower alkyl or optionally substituted lower alkylphenyl; and
R2 represents optionally substituted benzyl,
which process comprises the reaction of a compound of formula II

wherein R1 is as defined above, with a compound of formula III,
R2NH2xe2x80x83xe2x80x83III
wherein R2 is as defined above.
Alkyl groups that R1 may represent may be linear or branched. Suitable groups include linear or branched C1-6 alkyl, especially C1-4 alkyl groups such as t-butyl, n-propyl, i-propyl, ethyl and, especially, methyl groups.
Preferred alkylphenyl groups that R1 may represent include optionally substituted C1-3 alkylphenyl groups, such as optionally substituted benzyl groups. Alkylphenyl (including benzyl) groups that R1 and R2 may represent may be substituted on the alkyl part and/or on the phenyl part.
Optional substituents on R1 and R2 groups include halo (e.g. chloro and bromo), C1-6 (e.g. C1-4) alkyl (such as methyl), and C1-6 (e.g. C1-4) alkoxy (such as methoxy). Substituents on phenyl parts of alkylphenyl groups may be single or multiple and may be in any position relative to the alkyl part. Preferred points of substitution on phenyl rings include in the 4-position relative to the alkyl part. 4-Methoxy is an especially preferred substituent. Substituents on the alkyl parts of the alkylphenyl groups include, preferably, C1-6 (e.g. C1-4) alkyl, such as propyl, ethyl or methyl.
There is further provided a compound of formula I in which R1 represents optionally substituted lower alkylphenyl (e.g. benzyl or 4-methoxybenzyl) and R2 represents optionally substituted benzyl (e.g. benzyl or 4-methoxybenzyl).
There is further provided a compound of formula II in which R1 represents optionally substituted lower alkylphenyl (e.g. benzyl or 4-methoxybenzyl).
The skilled person will appreciate that the process of the invention involves two consecutive reactions, the first an amination of a compound of formula II using a compound of formula III, the second a cyclisation of an aminated intermediate to form a compound of formula I. In this respect, an aminated intermediate of formula Ia: 
wherein R1 and R2 are as hereinbefore defined may be isolated (or at least partially isolated) if desired. However, we prefer that the process of the invention is carried out as a one pot procedure.
The process of the invention may be carried out in an appropriate reaction solvent that does not interfere with the amination/cyclisation process. Appropriate reaction solvents include esters, such as ethyl acetate and iso-propyl acetate, ethers, such as tetrahydrofuran, polar aprotic solvents, such as acetonitrile, chlorinated solvents, such as dichloromethane, or mixtures thereof. Preferred solvents for the amination include polar aprotic solvents (especially acetonitrile), ethers (especially ethyl acetate and iso-propyl acetate). Preferred solvents for the cyclisation include polar aprotic solvents (especially acetonitrile).
The process of the invention may be carried out at an appropriate reaction temperature. Appropriate reaction temperatures for the amination are between room temperature (e.g. 20xc2x0 C.) and the reflux temperature of the solvent that is employed. Appropriate reaction temperatures for the cyclisation are between 35xc2x0 C. and the reflux temperature of the solvent that is employed, for example between 40xc2x0 C. and reflux. In the case of a one pot process of the invention, in which the solvent that is employed is acetonitrile, preferred reaction temperatures are between 50xc2x0 C. and reflux, especially reflux temperature, though the skilled person will appreciate that, even in a one-pot process, it is possible to initiate the reaction at or around room temperature and thereafter increase the temperature in order to promote the cyclisation.
The process of the invention may also be carried out in the presence of base, such as a carbonate of an alkali metal or an alkaline earth metal (e.g. potassium carbonate or calcium carbonate), and/or a catalyst, such as a source of iodine (e.g. an iodide of an alkali metal or an alkaline earth metal, such as sodium iodide or potassium iodide, or a quaternary ammonium iodide).
The process of the invention may be monitored using means that are well known to those skilled in the art. Appropriate reaction times will depend upon the degree and efficiency of conversion but are in the range 15 minutes to 48 hours, preferably 1 hour to 36 hours, more preferably 2 hours to 24 hours (for both the amination and cyclisation steps together, whether carried out separately or otherwise).
Appropriate concentrations of reactants, and proportion of reagents, may be determined readily by the skilled person. In any event, the skilled person will appreciate that reaction parameters, such as solvents, reagents, reaction times and reaction temperatures are interrelated, and will be able to devise and/or optimise appropriate parameters in accordance with routine techniques.
Work up, and isolation of compounds of formula I, may be carried out using routine techniques, such as those described hereinafter.
Compounds of formula II may be prepared for example by reaction of 4-chlorobutyryl chloride with bromine, followed by reaction of the brominated intermediate with an appropriate alkyl or alkylphenyl alcohol of formula IV,
R1OHxe2x80x83xe2x80x83IV
wherein R1 is as hereinbefore defined. This reaction may advantageously be carried out in a one pot procedure, that is to say the brominated intermediate need not be isolated.
The bromination may be carried out by adding bromine directly to 4-chlorobutyryl chloride at an appropriate reaction temperature e.g. at or around 90 to 120xc2x0 C., e.g. 100 to 110xc2x0 C. The esterification may be carried out at or around room temperature, optionally in the presence of an appropriate reaction solvent that does not interfere with the reaction, such as a lower (e.g. C6-12) alkane (e.g. heptane). Alternatively the alkyl alcohol of formula IV may be used as a solvent. This reaction may also optionally be carried out in the presence of a suitable base, such as sodium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, sodium hydroxide, potassium hydroxide or, preferably, calcium carbonate or potassium carbonate. Appropriate reaction times for the bromination will depend upon the degree and efficiency of conversion but are in the range 1 to 5 hours, preferably 2 to 4 hours. Appropriate reaction times for the esterification will also depend upon the degree and efficiency of conversion but are in the range 2 to 20 hours, preferably 3 to 15 hours.
When alkyl and alkylphenyl 2-bromo-4-chlorobutyrates are prepared in this way, we have found, advantageously, that there is no need to purify (e.g. by distillation) the butyrate compound before performing a subsequent cyclisation step as described herein.
Compounds of formula I may be deprotected using standard deprotection techniques (e.g. hydrogenolysis in the presence of a suitable catalyst, by acid or base hydrolysis or by a combination of these methods, for example as described hereinafter), resulting in AzeOH, e.g. as the racemic compound.
The skilled person will appreciate that, if desired or required, transesterification reactions may be performed at certain stages in the overall route for the preparation of compounds of formula I, or indeed after such compounds have been formed. For example compounds of formulae I and II may be transesterified, once formed, to other compounds of formulae I and II respectively, using known techniques (e.g. as described hereinafter).
In particular, we have found that compounds of formula I in which R1 represents optionally substituted benzyl may advantageously be deprotected in a one pot procedure, using standard deprotection techniques, such as those mentioned herein.
Compounds of formula I in which R1 represents optionally substituted benzyl may be prepared directly by reacting a compound of formula II in which R1 represents optionally substituted benzyl with a compound of formula III. Preferably, however, compounds of formula I in which R1 represents optionally substituted benzyl may be prepared by reacting a compound of formula II in which R1 represents lower alkyl (such as t-butyl, n-propyl, i-propyl, ethyl or, especially, methyl) with a compound of formula III, followed by transesterification of the resultant compound of formula I using standard techniques (e.g. using optionally substituted benzyl alcohol), for example as described hereinafter. We have found, surprisingly, that producing a compound of formula I in which R1 represents optionally substituted benzyl in the latter way greatly facilitates the overall process and results in an improved impurity profile for the resultant compound.
AzeOH formed after a deprotection reaction may, if desired, be resolved to give enantiomerically-pure D- and/or, particularly, L-AzeOH, using known resolution techniques, for example as described in international patent applications WO 97/02241 or WO 97/41084, the disclosures in which documents are hereby incorporated by reference. By xe2x80x9cenantiomerically-purexe2x80x9d AzeOH, we include any mixture of the enantiomers of AzeOH in which one enantiomer is present in a greater proportion than the other.
Thus, the process of the invention may be used as part of an overall synthesis for the production of AzeOH, D- and/or L-AzeOH, from 4-chlorobutyryl chloride. The AzeOH formed by way of the process of the invention may be utilised in a subsequent peptide coupling reaction. If enantiomerically-pure AzeOH is obtained by way of processes described in international patent applications WO 97/02241 and/or WO 97/41084, or by analogous processes, the skilled person will appreciate that it may be obtained by way of a diastereomerically-active tartrate salt. In such cases, enantiomerically-pure AzeOH need not be separately liberated/isolated before a subsequent coupling step is carried out.
The process of the invention has the advantage that it may be used in the production of AzeOH in a manner that means that the number of steps involved in the synthesis of the final compound is reduced.
Moreover, the process of the invention results in products (compounds of formula I) that are easy to extract, in a chemically pure form, from reaction solutions. Further, such product may be deprotected in situ (i.e. without the need to isolate it) to give AzeOH, in a manner that involves minimal loss of yield.
The process of the invention may also have the advantage that halogenated butyric acid derivatives may be cyclised to form four-membered rings comprising the azetidine-2-carboxylate moiety in higher yields, in higher chemical purity, in less time, more conveniently, at higher concentrations and at a lower cost, than when prepared via processes described in the prior art.